I did send BK Precision a message using their support site basically begging for a copy of the user manual. No word back yet. They have most of their user manuals online, but the 1590a is old enough to not be there. Possibly released prior to the invention of the pdf file format
First, the folks from this forum have proven themselves to be patient, smart and willing to help. Second, BK Precision has proven they are awesome because they were kind enough to scan the 1590A manual and send it to me. If anyone else from this forum happens to need a copy of the BK Precision 1590A user manual, or if anyone happens to duckduckgo for that in the future I'm making it available for download here:
https://web.tresorit.com/l#Bbu6d7SlnkdhGLxRH2oIjA
